High-mn steel and method for manufacturing same

ABSTRACT

Provided is a high-Mn steel which not only has high strength and excellent low-temperature toughness but also has excellent CTOD property at low temperatures. The high-Mn steel has a chemical composition containing, in mass %, C: 0.10-0.70%, Si: 0.05-0.50%, Mn: 20-30%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01-0.07%, Cr: 0.5-7.0%, Ni: 0.01% or more and less than 0.1%, Ca: 0.0005-0.0050%, N: 0.0050-0.0500%, O: 0.0050% or less, Ti: less than 0.0050%, and Nb: less than 0.0050%, the balance consisting of Fe and inevitable impurities, and a microstructure having austenite as a base phase, where the austenite has a grain size of 1 μm or more and a standard deviation of 9 μm or less.

TECHNICAL FIELD

This disclosure relates to a high-Mn steel suitable for a structure used in cryogenic environments, such as a tank for liquefied gas storage, and a method for manufacturing the same.

BACKGROUND

A structure for liquefied gas storage is used at cryogenic temperatures. Therefore, a steel sheet used for this type of structure is required to have not only high strength but also excellent toughness at cryogenic temperatures. For example, when a hot rolled steel sheet is used for liquefied natural gas storage, it is necessary to ensure excellent toughness at a temperature of −164° C., which is the boiling point of the liquefied natural gas, or lower. If the low-temperature toughness of the steel material is inferior, the safety as a structure for cryogenic storage may not be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material.

In response to this demand, austenitic stainless steel, 9% Ni steel, and 5000 series aluminum alloy, where austenite, which does not exhibit brittleness at cryogenic temperatures, is the main structure of the steel sheet, have conventionally been used. However, because of the high alloy cost and manufacturing cost, there has been a desire for a steel material that is inexpensive yet has excellent low-temperature toughness.

JP 2016-84529 A (PTL 1) and JP 2016-196703 A (PTL 2) propose using a high-Mn steel containing a large amount of Mn, which is a relatively inexpensive austenite-stabilizing element, as a structural steel in cryogenic environments, as a new steel material to replace conventional cryogenic steels.

That is, PTL 1 proposes controlling the carbide coverage of austenite crystal grain boundaries, and PTL 2 proposes controlling the austenite crystal grain size by a carbide coating as well as addition of Mg, Ca, and REM.

CITATION LIST Patent Literature

PTL 1: JP 2016-84529 A

PTL 2: JP 2016-196703 A

SUMMARY Technical Problem

However, in applications such as tanks for liquefied gas storage, it is required to have excellent fracture resistance under severe fracture conditions in which an initial crack becomes sharper, specifically, excellent CTOD property at low temperatures from the viewpoint of ensuring the safety of the tanks. Although PTL 1 and PTL 2 described above evaluate the low-temperature toughness by a Charpy impact test, they do not guarantee excellent CTOD property.

It could thus be helpful to provide a high-Mn steel which not only has high strength and excellent low-temperature toughness but also has excellent CTOD property at low temperatures. As used herein, the “high strength” means that the yield strength is 400 MPa or more, the “excellent low-temperature toughness” means that the absorbed energy vE-196 of a Charpy impact test at −196° C. is 100 J or more, and the “excellent CTOD property at low temperatures” means that the CTOD value at −165° C. is 0.25 mm or more.

Solution to Problem

We have conducted extensive research on methods for solving the problem with respect to high-Mn steels. As a result, we found the following a to b.

a. High-Mn steels do not develop brittle fractures even at cryogenic temperatures, and, if a fracture occurs, it is generated from crystal grain boundaries. Therefore, in order to improve the fracture resistance of high-Mn steels, it is effective to regulate the size of crystal grains in anticipation of reducing the area of crystal grain boundaries that are the starting point of fractures. b. In addition, realizing homogenization in conjunction with the above regulation of crystal grain size is more effective in improving the fracture resistance of high-Mn steels. c. As means for achieving the above a and b, it is appropriate to perform hot rolling and cooling under appropriate manufacturing conditions.

The present disclosure is based on the aforementioned findings and further studies. We thus provide the following.

1. A high-Mn steel comprising

-   -   a chemical composition containing (consisting of), in mass %,         -   C: 0.10% or more and 0.70% or less,         -   Si: 0.05% or more and 0.50% or less,         -   Mn: 20% or more and 30% or less,         -   P: 0.030% or less,         -   S: 0.0070% or less,         -   Al: 0.01% or more and 0.07% or less,         -   Cr: 0.5% or more and 7.0% or less,         -   Ni: 0.01% or more and less than 0.1%,         -   Ca: 0.0005% or more and 0.0050% or less,         -   N: 0.0050% or more and 0.0500% or less,         -   O: 0.0050% or less,         -   Ti: less than 0.0050%, and         -   Nb: less than 0.0050%,         -   the balance consisting of Fe and inevitable impurities, and     -   a microstructure having austenite as a base phase, where the         austenite has a grain size of 1 μm or more and a standard         deviation of 9 μm or less.

2. The high-Mn steel according to the above 1., wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of

-   -   Cu: 1.0% or less,     -   Mo: 2.0% or less,     -   V: 2.0% or less,     -   W: 2.0% or less,     -   Mg: 0.0005% or more and 0.0050% or less, and     -   REM: 0.0010% or more and 0.0200% or less.

3. A method of manufacturing a high-Mn steel, comprising heating a steel material having the chemical composition according to the above 1. or 2. to a temperature range of 1100° C. or higher and 1300° C. or lower, then subjecting the steel material to hot rolling where a rolling finish temperature is 750° C. or higher and lower than 950° C. and an average rolling reduction for one pass is 9% or more, and then subjecting the hot rolled material to cooling where an average cooling rate from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower is 1.0° C./s or more.

Advantageous Effect

According to the present disclosure, it is possible to provide a high-Mn steel having excellent CTOD property and low-temperature toughness especially at cryogenic temperatures. Therefore, by using the high-Mn steel of the present disclosure, it is possible to realize an improvement in safety and product life of a steel structure used in cryogenic environments, such as a tank for liquefied gas storage, which exhibits remarkable industrial effects.

DETAILED DESCRIPTION

The following describes the high-Mn steel of the present disclosure in detail.

[Chemical Composition]

First, the chemical composition of the high-Mn steel of the present disclosure and reasons for limitation will be described. Note that the unit “%” of each component is “mass %” unless otherwise specified. C: 0.10% or more and 0.70% or less

C is an inexpensive austenite-stabilizing element and is an important element in obtaining austenite. To obtain this effect, the C content needs to be 0.10% or more. On the other hand, when the C content exceeds 0.70%, Cr carbides are excessively formed, and the low-temperature toughness is deteriorated. Therefore, the C content is 0.10% or more and 0.70% or less. The C content is preferably 0.20% or more. The C content is preferably 0.60% or less.

Si: 0.05% or more and 0.50% or less

Si is an element that acts as a deoxidizing material. It not only is necessary for steelmaking but also dissolves in steel to increase the strength of a steel sheet by solid solution strengthening. To obtain these effects, the Si content needs to be 0.05% or more. On the other hand, when the Si content exceeds 0.50%, the weldability is deteriorated and the low-temperature toughness, especially the toughness at cryogenic temperatures is lowered. Therefore, the Si content is 0.05% or more and 0.50% or less. The Si content is preferably 0.07% or more and 0.50% or less.

Mn: 20% or more and 30% or less

Mn is a relatively inexpensive austenite-stabilizing element. Mn is an important element for the present disclosure to achieve both the strength and the toughness at cryogenic temperatures. To obtain this effect, the Mn content needs to be 20% or more. On the other hand, when the content exceeds 30%, the effect of improving low-temperature toughness saturates, leading to an increase in alloy cost. In addition, the weldability and the cuttability deteriorate. Further, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, the Mn content is 20% or more and 30% or less. The Mn content is preferably 23% or more. The Mn content is preferably 28% or less.

P: 0.030% or less

When P is contained in excess of 0.030%, it segregates at grain boundaries and becomes a starting point of stress corrosion cracking. Therefore, the upper limit is set to 0.030%, and the P content is desirably as low as possible. Thus, the P content is 0.030% or less. Note that the P content is desirably 0.002% or more, because excessive reduction of P content increases refining cost and is economically disadvantageous. The P content is preferably 0.005% or more. The P content is preferably 0.028% or less. The P content is more preferably 0.024% or less.

S: 0.0070% or less

S is an element that deteriorates low-temperature toughness and base metal ductility. Therefore, the upper limit is set to 0.0070%, and the S content is desirably as low as possible. Thus, the S content is 0.0070% or less. Note that the S content is desirably 0.001% or more, because excessive reduction of S content increases refining cost and is economically disadvantageous. The S content is preferably 0.0020% or more. The S content is preferably 0.0060% or less.

Al: 0.01% or more and 0.07% or less

Al acts as a deoxidizer and is most commonly used in a molten steel deoxidation process of a steel sheet. To obtain this effect, the Al content needs to be 0.01% or more. On the other hand, when the Al content exceeds 0.07%, Al is mixed into a weld metal part during welding and deteriorates the toughness of the weld metal. Therefore, the Al content is 0.07% or less. Thus, the Al content is 0.01% or more and 0.07% or less. The Al content is preferably 0.02% or more. The Al content is preferably 0.06% or less.

Cr: 0.5% or more and 7.0% or less

Cr is an element that stabilizes austenite when added in an appropriate amount and is an element effective in improving low-temperature toughness and base metal strength. To obtain these effects, the Cr content needs to be 0.5% or more. On the other hand, when the content exceeds 7.0%, the low-temperature toughness and the stress corrosion cracking resistance are deteriorated due to formation of Cr carbides. Therefore, the Cr content is 0.5% or more and 7.0% or less. The Cr content is preferably 1.0% or more. The Cr content is preferably 6.7% or less. The Cr content is more preferably 1.2% or more. The Cr content is more preferably 6.5% or less. In order to further improve the stress corrosion cracking resistance, the content is still more preferably 2.0% or more and 6.0% or less.

Ni: 0.01% or more and less than 0.1%

Ni has the effect of improving low-temperature toughness. However, minimizing the necessary alloy cost is an important viewpoint in designing the composition of the present disclosure, and from this viewpoint, the Ni content is 0.01% or more and less than 0.1%. Examples of austenitic steels having excellent low-temperature toughness include stainless steels such as SUS304 and SUS316. However, a large amount of Ni is added in these steels to optimize the Ni equivalent and the Cr equivalent as an alloy design for obtaining an austenitic structure. Compared with these steels, the present disclosure is an austenitic material whose price is lowered by minimizing necessary Ni. Note that the minimization of necessary Ni is realized by optimizing the addition amount of Mn. The Ni content is preferably 0.03% or more. The Ni content is preferably 0.07% or less.

Ca: 0.0005% or more and 0.0050% or less

Ca improves ductility, toughness and sulfide stress corrosion cracking resistance by controlling the morphology of inclusions described below. In addition, Ca suppresses the deterioration of hot ductility and is effective in reducing the occurrence of cracks in cast steel. To obtain these effects, the Ca content needs to be 0.0005% or more. On the other hand, when the Ca content exceeds 0.0050%, the ductility, toughness, and sulfide stress corrosion cracking resistance may be rather deteriorated, and the effect of suppressing the deterioration of hot ductility may saturate. Therefore, the Ca content is 0.0005% or more and 0.0050% or less. The Ca content is preferably 0.0010% or more. The Ca content is preferably 0.0045% or less.

N: 0.0050% or more and 0.0500% or less

N is an austenite-stabilizing element and is an element effective in improving low-temperature toughness. To obtain these effects, the N content needs to be 0.0050% or more. On the other hand, when the content exceeds 0.0500%, nitrides or carbonitrides are coarsened and the toughness is deteriorated. Therefore, the N content is 0.0050% or more and 0.0500% or less. The N content is preferably 0.0060% or more. The N content is preferably 0.0400% or less.

O: 0.0050% or less

O deteriorates low-temperature toughness due to formation of oxides. Therefore, the O content is in the range of 0.0050% or less. The O content is preferably 0.0045% or less. Note that the O content is desirably 0.0003% or more, because excessive reduction of O content increases refining cost and is economically disadvantageous.

Ti and Nb contents each suppressed to less than 0.005%

Ti and Nb form high-melting carbonitrides in steel and suppress the coarsening of crystal grains, and as a result, they become a starting point of fractures and propagation path of cracks. In particular, they hinder the microstructure control for enhancing the low-temperature toughness and improving the ductility in the high-Mn steel. Therefore, the contents of Ti and Nb must be suppressed intentionally. That is, Ti and Nb are components inevitably mixed from raw materials and the like, and they are generally mixed in the ranges of Ti: 0.005% to 0.010% and Nb: 0.005% to 0.010%. Therefore, it is necessary to avoid the inevitable mixing of Ti and Nb and to suppress the content of each of Ti and Nb to less than 0.005% according to a method described below. By suppressing the content of each of Ti and Nb to less than 0.005%, it is possible to eliminate the above-mentioned adverse effects of carbonitrides and to ensure excellent low-temperature toughness and ductility. The contents of Ti and Nb are preferably 0.003% or less.

The balance other than the above essential components is iron and inevitable impurities. Examples of the inevitable impurities here include H, and a total of 0.01% or less is acceptable.

In order to further improve strength and low-temperature toughness, the following elements can be contained as necessary in addition to the above essential components in the present disclosure.

At least one of Cu: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005% to 0.0050%, or REM: 0.0010% to 0.0200% Cu: 1.0% or less, Mo, V, W: each 2.0% or less

Cu, Mo, V and W contribute to the stabilization of austenite and to the improvement of base metal strength. To obtain these effects, the contents of Cu, Mo, V and W are preferably 0.001% or more. On the other hand, when the Cu content exceeds 1.0% and the contents of Mo, V and W each exceed 2.0%, coarse carbonitrides are formed, which may be a starting point of fractures, and the manufacturing cost also increases. Therefore, when these alloying elements are contained, the contents are 1.0% or less for Cu and 2.0% or less for Mo, V and W. The contents are preferably 0.003% or more. Further, the contents of Mo, V and W are preferably 1.7% or less. The contents of Mo, V and W are more preferably 1.5% or less.

Mg: 0.0005% to 0.0050%, and REM: 0.0010% to 0.0200%

Mg and REM are useful elements for controlling the morphology of inclusions and can be contained as necessary. Controlling the morphology of inclusions means making expanded sulfide-based inclusions into granular inclusions. By controlling the morphology of inclusions, the ductility, toughness and sulfide stress corrosion cracking resistance are improved. To obtain these effects, the Ca and Mg contents are preferably 0.0005% or more, and the REM content is preferably 0.0010% or more. On the other hand, when any of these elements is contained in a large amount, the amount of nonmetallic inclusions increases, and the ductility, toughness, and sulfide stress corrosion cracking resistance may rather be deteriorated. In addition, it may be economically disadvantageous. Therefore, when Mg is contained, the content is 0.0005% to 0.0050%, and when REM is contained, the content is 0.0010% to 0.0200%. The Mg content is preferably 0.0010% or more. The Mg content is preferably 0.0040% or less. The REM content is preferably 0.0020% or more. The REM content is preferably 0.0150% or less.

[Microstructure]

Microstructure Having Austenite as a Base Phase

When the crystal structure of a steel material is a body-centered cubic structure (bcc), the steel material is not suitable for use in low-temperature environments because it may cause brittle fractures in low-temperature environments. In consideration of the use in low-temperature environments, the base phase of the steel material should be an austenitic structure where the crystal structure is a face-centered cubic structure (fcc). As used here, “austenite as a base phase” means that the austenite phase has an area ratio of 90% or more. The remainder other than the austenite phase is a ferrite phase or a martensite phase. Of course, the austenite phase may be 100%.

Austenite grain size: 1 μm or more

Because the high-Mn steel has a microstructure having austenite as a base phase, brittle fractures do not occur even at cryogenic temperatures, and, if a fracture occurs, it is generated from crystal grain boundaries. It is advantageous to reduce the area of crystal grain boundaries, which are the starting point of fractures, to improve the fracture resistance of the high-Mn steel. Therefore, it is important that the austenite grain size be 1 μm or more. This is because, when the grain size is less than 1 μm, the increasing amount of grain boundary area increases, which increases the number of locations where fractures occur. It is preferably 2 μm or more.

Standard deviation of austenite: 9 μm or less

Realizing homogenization in conjunction with the regulation of crystal grain size is effective in further improving the fracture resistance of the high-Mn steel. That is, in a mixed-grain-size microstructure, a wide grain size distribution from coarse crystal grains to fine crystal grains results in containing of crystal grains of less than 1 μm, and especially when the standard deviation exceeds 9 μm, the tendency is remarkable. Therefore, it is necessary to avoid a mixed-grain-size microstructure having a standard deviation of more than 9 μm.

[Manufacturing Method]

During the manufacture of the high-Mn steel of the present disclosure, first, the steel material may be a molten steel having the above-described chemical composition obtained with a known smelting method such as a converter or an electric furnace. In addition, secondary refinement may be performed in a vacuum degassing furnace. At that time, in order to limit Ti and Nb, which hinder the control of a preferable microstructure, to the above-described ranges, it is necessary to avoid inevitable mixing from raw materials and the like and take measures to reduce the contents thereof. For example, by lowering the basicity of slag in the refining stage, these alloys are concentrated and discharged into the slag, which reduces the concentration of Ti and Nb in a final slab product. Alternatively, a method of blowing oxygen to oxidize the Ti and Nb and floating and separating the alloy of Ti and Nb in reflux may also be used. Subsequently, it is preferable to obtain a steel material such as a slab having a predetermined size with a known casting method such as a continuous casting method or an ingot casting method. It is also acceptable to subject the slab after continuous casting to blooming to obtain a steel material.

The following specifies the manufacturing conditions for making the above steel material into a steel material having excellent low-temperature toughness.

Steel material heating temperature: 1100° C. or higher and 1300° C. or lower

The heating temperature before hot rolling is 1100° C. or higher to increase the crystal grain size of the microstructure of the steel material. However, when the temperature exceeds 1300° C., partial melting may start. Therefore, the upper limit of the heating temperature is set to 1300° C. The temperature control here is based on the surface temperature of the steel material.

Rolling finish temperature: 750° C. or higher and lower than 950° C.

The steel material (steel ingot or slab) is subjected to hot rolling after the heating. In order to obtain coarse crystal grains, it is preferable to increase the cumulative rolling reduction at high temperatures. That is, performing hot rolling at a low temperature makes the microstructure fine and causes excessive working strain. As a result, the low-temperature toughness is deteriorated. Therefore, the lower limit of the rolling finish temperature is set to 750° C. On the other hand, when the finish temperature is in the range of 950° C. or higher, the crystal grain size becomes excessively coarse, and a desired yield strength cannot be obtained. Therefore, it is necessary to perform the final finish rolling of one or more passes at a temperature of lower than 950° C. It is preferably 900° C. or lower.

Average rolling reduction for one pass: 9% or more

During the hot rolling, in order to realize the homogenization of austenite grain size and control the crystal grain size to 1 μm or more, it is effective to promote the recrystallization of austenite, and it is important to have an average rolling reduction for one pass of 9% or more during the hot rolling. It is preferably 11% or more.

Average cooling rate from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower: 1.0° C./s or more

Cooling is immediately performed after the hot rolling. If the steel sheet after hot rolling is cooled slowly, formation of precipitates is promoted and the low-temperature toughness is deteriorated. The formation of these precipitates can be suppressed by cooling the steel sheet at a cooling rate of 1.0° C./s or more. Excessive cooling distorts the steel sheet and lowers the productivity. Therefore, the upper limit of the cooling start temperature is set to 900° C. For the above reasons, in the cooling after the hot rolling, the average cooling rate at the steel sheet surface from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower is 1.0° C./s or more. On the other hand, from the viewpoint of industrial production, the average cooling rate is preferably 200° C./s or less.

EXAMPLES

The following provides a more detailed explanation of the present disclosure through examples. However, the present disclosure is not limited to the following examples.

Steel slabs having the chemical composition listed in Table 1 were prepared by a process for refining with converter and ladle and continuous casting. Next, the steel slabs thus obtained were subjected to blooming and hot rolling under the conditions listed in Table 2 to obtain steel sheets having a thickness of 10 mm to 30 mm. The steel sheets thus obtained were subjected to tensile property, toughness and microstructure evaluation as described below.

(1) Tensile Test Property

A JIS No. 5 tensile test piece was collected from each steel sheet thus obtained, and the tensile test piece was subjected to a tensile test according to the provisions of JIS Z2241 (1998) to investigate the tensile test property. In the present disclosure, a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more were determined to be excellent in tensile properties. Further, elongation of 40% or more was determined to be excellent in ductility.

(2) Low-Temperature Toughness

A Charpy V-notch test piece was collected from a direction parallel to the rolling direction at a position at a depth of one-fourth of the sheet thickness from the surface of each steel sheet having a thickness of more than 20 mm (hereinafter referred to as “position of sheet thickness×¼”), or a position at a depth of half of the sheet thickness of each steel sheet having a thickness of 20 mm or less (hereinafter referred to as “position of sheet thickness×½”) according to the provisions of JIS Z2202 (1998). Three

Charpy impact tests were performed on each steel sheet according to the provisions of JIS Z2242 (1998) to determine the absorbed energy at −196° C., thereby evaluating the base metal toughness. In the present disclosure, when the average of three absorbed energy (vE−196) values was 100 J or more, it was determined to be excellent in base metal toughness.

(3) CTOD Value Evaluation

A CTOD test piece was collected from a direction parallel to the rolling direction at the position of sheet thickness×½ of the steel sheet, and two or three tests were conducted at −165° C. to evaluate the average value. In the present disclosure, a CTOD value of 0.25 mm or more was determined to be excellent in fracture resistance.

(4) Microstructure

Electron backscatter diffraction (EBSD) analysis was performed on a L cross section at the position of sheet thickness×¼ of the steel sheet. Two or three visual fields of 200 μm×200 μm were selected arbitrarily and observed, and the minimum value of austenite crystal grain size in each visual field was measured. In addition, the standard deviation of the austenite grain size was evaluated from the distribution of the area ratio of each crystal grain size using the results of the EBSP analysis. All the crystal grain sizes thus obtained were taken as a population, a variance that is a sum of squares of the difference between each individual value and the average value was obtained, and a square root of the variance was obtained to determine the standard deviation.

The evaluation results thus obtained are listed in Table 3.

It has been confirmed that the high-Mn steel of the present disclosure satisfies the above-mentioned desired performance (the yield strength of base metal is 400 MPa or more, the average value of absorbed energy (vE-196) is 100 J or more with respect to the low-temperature toughness, and the average value of CTOD value is 0.25 mm or more). On the other hand, Comparative Examples, which are outside the scope of the present disclosure, do not satisfy at least one of the above-mentioned desired performance of yield strength, low-temperature toughness, and CTOD value.

TABLE 1 Steel Chemical composition (mass %) No. C Si Mn P S Al Cr O N Nb Ti A 0.162 0.22 29.6 0.019 0.0049 0.055 4.52 0.0019 0.0176 0.001 0.001 B 0.664 0.09 21.9 0.013 0.0051 0.031 2.66 0.0047 0.0385 0.002 0.001 C 0.420 0.41 23.5 0.021 0.0035 0.029 4.46 0.0021 0.0233 0.003 0.002 D 0.343 0.49 20.8 0.021 0.0066 0.041 3.24 0.0035 0.0189 0.001 0.003 E 0.291 0.37 28.5 0.026 0.0025 0.064 1.78 0.0024 0.0237 0.002 0.001 F 0.459 0.28 26.7 0.016 0.0052 0.040 6.21 0.0047 0.0355 0.002 0.002 G 0.332 0.43 23.4 0.019 0.0050 0.052 2.44 0.0029 0.0194 0.003 0.001 H 0.402 0.22 20.6 0.021 0.0044 0.032 1.25 0.0039 0.0065 0.003 0.002 I 0.312 0.15 25.5 0.019 0.0030 0.044 5.42 0.0023 0.0145 0.004 0.001 J 0.415 0.35 24.0 0.018 0.0032 0.031 4.13 0.0019 0.0206 0.001 0.002 K 0.590 0.35 26.4 0.014 0.0033 0.036 4.34 0.0016 0.0088 0.002 0.002 L 0.929 0.40 20.1 0.028 0.0055 0.032 4.11 0.0042 0.0304 0.004 0.003 M 0.105 0.03 24.4 0.022 0.0029 0.041 5.07 0.0031 0.0298 0.001 0.002 N 0.128 0.44 16.9 0.025 0.0049 0.038 2.10 0.0040 0.0463 0.002 0.002 O 0.220 0.50 23.5 0.049 0.0067 0.042 0.93 0.0044 0.0321 0.001 0.001 P 0.315 0.32 27.7 0.030 0.0091 0.025 1.35 0.0028 0.0059 0.001 0.003 Q 0.498 0.29 23.9 0.018 0.0055 0.094 3.49 0.0029 0.0250 0.003 0.002 R 0.287 0.44 26.5 0.015 0.0042 0.063 7.97 0.0037 0.0187 0.002 0.001 S 0.433 0.43 26.1 0.025 0.0035 0.052 6.22 0.0079 0.0224 0.004 0.003 T 0.367 0.09 24.6 0.029 0.0048 0.024 3.27 0.0029 0.0755 0.002 0.001 Steel Chemical composition (mass %) No. V Cu Ni Mo W Ca Mg REM Remarks A — — 0.05 — — 0.0011 — — Example B — — 0.08 — — 0.0023 — — Example C — — 0.02 — — 0.0005 — — Example D 0.07 — 0.03 0.46 — 0.0034 — — Example E — — 0.01 — 0.08 0.0042 — — Example F — — 0.05 — — 0.0021 — — Example G — — 0.04 — — 0.0018 0.0016 — Example H — — 0.09 — — 0.0011 — 0.0031 Example I — — 0.06 — — 0.0024 — — Example J — 0.58 0.05 — — 0.0028 — — Example K — — 0.07 — — 0.0031 — — Example L — — 0.03 — — — — — Comparative Example M — — — — — 0.0002 — — Comparative Example N — — 0.04 — — — — — Comparative Example O — — 0.01 — — — — — Comparative Example P — — 0.02 — — — — — Comparative Example Q — — 0.15 — — — — — Comparative Example R — — 0.02 — — — — — Comparative Example S — — 0.03 — — 0.0061 — — Comparative Example T — — 0.05 — — — — — Comparative Example

TABLE 2 Hot rolling method Average Cooling rate rolling until the range of Plate Slab heating Rolling finish reduction Cooling start 300° C. or higher and Sample Steel thickness temperature temperature for one pass temperature 650° C. or lower No. No. (mm) (° C.) (° C.) (%) (° C.) (° C./s) Remarks 1 A 31 1180 857 11 830 9 Example 2 B 25 1150 830 12 800 10 Example 3 C 11 1160 785 14 730 12 Example 4 D 20 1100 796 12 764 10 Example 5 E 25 1130 855 12 825 10 Example 6 F 14 1130 842 14 787 11 Example 7 G 9 1250 883 15 819 16 Example 8 H 10 1220 766 14 707 13 Example 9 I 15 1150 826 13 770 3 Example 10 J 30 1150 848 10 821 2 Example 11 C 10 1100 758 14 708 12 Example 12 J 12 1150 775 13 723 11 Example 12 L 20 1250 779 10 747 9 Comparative Example 13 M 30 1250 890 11 863 8 Comparative Example 14 N 14 1120 825 13 770 12 Comparative Example 15 O 25 1120 858 11 828 10 Comparative Example 16 P 20 1180 807 11 764 5 Comparative Example 17 Q 10 1180 773 12 714 11 Comparative Example 18 R 20 1150 801 11 764 9 Comparative Example 19 S 15 1150 810 12 754 10 Comparative Example 20 T 30 1130 845 9 821 2 Comparative Example 21 C 12 1180 673 10 618 11 Comparative Example 22 D 19 1200 804 11 772 0.4 Comparative Example 23 A 31 1200 759 7 732 9 Comparative Example 24 E 25 1030 848 12 818 10 Comparative Example 25 D 10 1100 763 6 716 11 Comparative Example 26 B 25 1130 993 10 955 10 Comparative Example

TABLE 3 Absorbed Minimum Standard energy value of deviation of Yield Tensile Total at −196° C. CTOD Sample Steel γ grain size γ grain size strength strength elongation (vE_(−196° C.)) value No. No. (μm) (μm) (MPa) (MPa) (%) (J) (mm) Remarks 1 A 5.2 7.6 418 850 69 133 0.41 Example 2 B 3.4 7.4 554 941 58 118 0.39 Example 3 C 2.6 5.7 566 969 62 123 0.36 Example 4 D 2.9 7.1 559 952 55 115 0.36 Example 5 E 5.5 7.3 489 886 64 138 0.42 Example 6 F 6.1 6.2 453 854 67 139 0.44 Example 7 G 5.9 4.9 418 936 62 126 0.44 Example 8 H 3.1 5.5 512 968 61 104 0.37 Example 9 I 5.7 6.6 447 849 66 131 0.43 Example 10 J 4.8 7.8 407 847 69 136 0.40 Example 11 C 1.9 5.4 575 978 58 115 0.36 Example 12 J 2.8 5.8 557 952 63 124 0.37 Example 12 L 2.3 7.9 622 970 51 58 0.21 Comparative Example 13 M 4.9 7.4 363 818 70 127 0.40 Comparative Example 14 N 5.5 6.8 443 868 68 36 0.17 Comparative Example 15 O 4.7 7.3 447 878 64 72 0.33 Comparative Example 16 P 4.8 7.5 529 940 59 79 0.36 Comparative Example 17 Q 2.9 6.1 502 957 61 89 0.37 Comparative Example 18 R 4.5 7.3 518 935 60 48 0.34 Comparative Example 19 S 5.2 6.8 455 850 67 83 0.40 Comparative Example 20 T 4.3 7.9 417 853 68 75 0.40 Comparative Example 21 C 1.7 6.5 667 1057 49 42 0.29 Comparative Example 22 D 2.5 7.3 547 943 56 67 0.34 Comparative Example 23 A 0.7 9.8 534 964 59 103 0.15 Comparative Example 24 E 3.5 7.4 476 875 63 69 0.36 Comparative Example 25 D 0.4 10.5 569 970 60 119 0.12 Comparative Example 26 B 4.9 7.5 375 795 70 112 0.37 Comparative Example 

1. A high-Mn steel comprising a chemical composition containing, in mass %, C: 0.10% or more and 0.70% or less, Si: 0.05% or more and 0.50% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 0.5% or more and 7.0% or less, Ni: 0.01% or more and less than 0.1%, Ca: 0.0005% or more and 0.0050% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: less than 0.0050%, and Nb: less than 0.0050%, the balance consisting of Fe and inevitable impurities, and a microstructure having austenite as a base phase, where the austenite has a grain size of 1 μm or more and a standard deviation of 9 μm or less.
 2. The high-Mn steel according to claim 1, wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of Cu: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0010% or more and 0.0200% or less.
 3. A method of manufacturing a high-Mn steel, comprising heating a steel material having the chemical composition according to claim 1 to a temperature range of 1100° C. or higher and 1300° C. or lower, then subjecting the steel material to hot rolling where a rolling finish temperature is 750° C. or higher and lower than 950° C. and an average rolling reduction for one pass is 9% or more, and then subjecting the hot rolled material to cooling where an average cooling rate from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower is 1.0° C./s or more.
 4. A method of manufacturing a high-Mn steel, comprising heating a steel material having the chemical composition according to claim 2 to a temperature range of 1100° C. or higher and 1300° C. or lower, then subjecting the steel material to hot rolling where a rolling finish temperature is 750° C. or higher and lower than 950° C. and an average rolling reduction for one pass is 9% or more, and then subjecting the hot rolled material to cooling where an average cooling rate from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower is 1.0° C./s or more. 